VIRTIS spectral images of the Earth
The VIRTIS hyperspectral imager aboard Rosetta took these pictures between 14:00-14:25 UT on 13 November 2009 during the mission's third Earth swingby. At the time (several hours after closest approach) the spacecraft was at about 230 000 km from the Earth's surface and receding again from the planet.
One of the channels of the VIRTIS instrument is the Mapper, a hyperspectral imager that collects image data simultaneously in 864 narrow, adjacent spectral bands (colours). The spectral information is used in studies of the composition of the observed areas, while the combined information from the image and spectral data provide two-dimensional compositional maps.
The first image (VIS-1) has been taken over the American continents on 13 November at 14:00 UT in the visible-near infrared spectral range (350-1000nm). It has a spatial resolution of about 50 km. The selected colours are those usually adopted in RGB imaging (with red, green and blue at 0.7 µm, 0.55 µm and 0.44 µm respectively). The clouds cover almost all the Earth, with only few areas of ground in North and South America visible. On the top-right the westernmost coast of Africa can also be seen.
The second figure (VIS-2) shows the same image as VIS-1, but now the colours have been selected to give maximum contrast to the various components: ground, clouds and sea. North and South America are clearly seen in this false-colour picture centred on the Gulf of Mexico. Additionally, the contrast between clouds and sea is increased. This enhancement is obtained by using a combination of colours with the blue (B), green (G) and red (R) channels respectively at 0.474 µm (for the sea water), 0.785 µm and 1.0 µm (to increase the contrast of the land).
A more interesting type of contrast enhancement can be done using selected spectral features. The chlorophyll spectral band, for instance, provides an immediate means to distinguish between ground as opposed to either sea or clouds. In figure VIS-3 the distribution of the chlorophyll over the visible hemisphere is shown. The colour scale is such that maximum chlorophyll abundance is represented in green. The black patches over several regions of South America are caused by dense clouds that completely mask the ground.
The Earth in the infrared spectral range is seen in the figure IR-1, which was made using a combination of colours with the R, G and B channels selected at 4.92 µm, 2.25 µm and 1.20 µm respectively. In this spectral range the Earth has a completely different appearance than in the visible as VIRTIS-M is sensitive both to the reflected solar radiation and to the emitted thermal radiation. This is the reason why the night side shows up in this image. The cyan areas are related to high altitude clouds, which are particularly bright in the near infrared range on the dayside. Landforms appear pink.
If we limit to the thermal emission at 5.0 µm, figure IR-2, the Earth looks fairly uniform on the day and night side. On the right-hand side of the image is the colour scale that was used corresponding to the measured radiance at 5.0 µm in MKS units. The warmest areas (in orange-red) are on the dayside and in the equatorial region, while the coldest areas (in violet) are the top of the clouds and the region in the top-left quadrant of the image which includes Canada and the Arctic.
VIRTIS is the Visible, InfraRed and Thermal Imaging Spectrometer of the ROSETTA mission and covers a wide spectral range which extends from the UV to the thermal infrared (0.25-5.0 µm). The instrument, led by an Italian PI Prof. Angioletta Coradini, is composed of 2 independent channels: the Mapping Spectrometer built in Italy (INAF-IFSI and INAF-IASF, Rome; Galileo Avionica, Florence and Italian Space Agency, ASI) and the High Resolution Spectrometer built in France (Observatoire de Paris and CNES). The instrument electronics have been provided by the Institut für Planetenforschung - DLR.